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In part because that venom is exerting selective pressure on their intended prey, and they end up in an evolutionary "arms race". Over the generations, some prey species evolve more and more resistance to the venom, and in response the snake needs to produce more potent venom just to be able to kill its food. You go back and forth like that for a couple hundred thousand years and eventually the snake is terrifyingly poisonous venomous*.

Meanwhile the rest of the animal world hasn't had any pressure to evolve resistance to the same level, so for any animal that hasn't been in this arms race the potency of the venom is absolutely overkill. But somewhere out there is a tiny rodent or bird or something which the snake likes to eat, which is almost (comparatively) immune to the venom.

E: venomous, not poisonous. Thank you /u/ShadowKing94 for pointing this out

Simple; to make damn sure that whatever they attack dies, increasing their chance of eating and thus surviving. IIRC, venom is pretty cheap (biologically) to produce, so make a bunch and use as much as possible to ensure death.

Most estimates of venom deadliness come with some small lettering attached: Snake X could kill an elephant (if all the venom present in the glands is injected directly into the bloodstream)

But snake X might not use all its stored venom, might miss with one fang, hit a bone or tissue that doesn't spread the venom fast enough ect ect. so they go all-in in toxicity so that even a mild bite is fatal quickly. Making more potent venom is relatively easy and so is starving to death if your venom isn't potent enough.

I suspect that "quickly" is a very important aspect here. If a snake's prey runs off and dies miles away, a few hours after being bitten, it doesn't help the snake much.
Likewise, a fatal bite to a predator doesn't do the snake any good if the predator has time to kill the snake before succumbing to the venom.

Both your comment and the one you replied to imply that snakes have control over how much venom they inject. The Vipirid species (for one) does not control how much venom they inject, it is simply an accident based on a number of factors surrounding the strike.

Evolution doesn't work that way. Things don't evolve to be "as [x] as possible" without some pressure to push it there. Expressing a trait more than is necessary is wasteful and unnecessarily costly (even if the cost is minimal), so evolution instead tends to produce traits that are just good enough. /u/mrtherussian has the correct answer.

Yep, and continuing your thought a bit, once that new "just good enough" trait is rendered "not good enough" anymore by selection pressures (be it on the predator or Prey, using the snake example), evolution will try once again to make it (or maybe some other trait) "just good enough" again, to return the advantage. Classic red queen/evolutionary arms race situation.

One thing that no one else mentioned is scaling. Smaller animals usually have much higher relative metabolic rates than larger animals, and thus may need larger doses (per unit of body weight) than larger animals. Researchers in the 60's found this out the hard way when they were testing the effects of LSD on elephants. They based the dosage they gave to the elephant on dosages given to smaller animals (relative to body mass), but because elephants have much lower relative metabolic rates than small animals do, that dosage ended up being lethal and killed the elephant they were testing.

I read an article about an evolutionary battle between predator and prey wherein it is thought that the venomous predator had to develop increasingly toxic venom to counteract the adaptation to it that the preferred prey was developing. I can't find it now, though I also found this discussion of the evolution of venom for defense against predators of the snake:

Many venomous snakes have very fine grained control over the quantity of venom injected. Just because they deliver a human lethal quantity of venom when biting a human doesn't mean the do the same when biting a rat.

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This is just a guess, but maybe because venom toxicity tends to be catalytic (enzymatic in nature) that often triggers other cellular pathways that multiply the effect. So it is more of a case that it is a very potent substance in very small quantitates and there is very little energy cost to making plenty and it would actually be tricky to titrate it down to just being enough.